This invention relates to the electroless metal plating of plastics employing an alkaline permanganate solution pretreatment process and, more particularly, to improving the efficiency and operating life of the permanganate step of the metal plating process.
The metal plating of plastic parts is well-known to be of considerable commercial importance. An important metal plating process is the preparation of electronic circuit boards which requires the electroless plating of a conductive metal layer, usually copper, onto the plastic substrate of the board, such as epoxy, and for convenience the following description will relate specifically to this process.
These boards vary in design and may have a copper layer on each surface of the epoxy (two-sided boards) or they can be multi-layer boards which have a plurality of inter-leaved parallel planar copper and epoxy layers. In both type boards through-holes are drilled in the board and metal plated to facilitate connection between the circuits on the copper layers.
The problems in plating either the through-holes or other plastic parts of the board are well-known in the art and a number of methods have been developed to improve the adhesion of the metal plating to the epoxy. These methods generally employ oxidants to etch the surface of the plastic prior to plating and include chromic acid, sulfuric acid and acidic and alkaline permanganate solutions. The toxicity of the chromium compounds and their potential hazards as water pollutants and the safety precautions needed with sulfuric acid have increased the commercial use of permanganate solutions, particularly alkaline permanganate solutions, and a number of patents have been granted in this area such as U.S. Pat. Nos. 3,652,351; 4,042,729; 4,054,693; 4,592,852; and 4,629,636; the disclosures of the patents being incorporated by reference.
The present invention relates to improving the operating life of the permanganate bath since, during use, the permanganate ions are reduced and depleted forming manganate ions and manganese dioxide. To maintain activity the bath must either be regenerated by the addition of permanganate ions or, when the solution becomes spent and cannot be regenerated because of the build-up contaminants, the bath must be replaced. Both procedures are time consuming and are not cost effective and U.S. Pat. Nos. 4,042,729; 4,529,852 and 4,629,636 are specifically addressed to this problem.
U.S. Pat. No. 4,042,729 provides a means for rejuvenating the bath to remove organics and produce permanganate by disproportionation of manganate ions comprising bubbling CO.sub.2 into the bath to lower the pH from 12.5 to 11 to 11.5, heating to form permanganate ions and manganese dioxide from the manganate ions, cooling, precipitating carbonates and filtering to remove the manganese dioxide and carbonates. The preparation and control of the compositions and rejuvenating procedures of this patent are unwieldly and time consuming however.
U.S. Pat. Nos. 4,592,852 and 4,629,636, which are owned by the assignee of the present invention, provide a significantly enhanced permanganate treatment process. Basically, the alkaline permanganate etching compositions comprise permanganate ions and a secondary oxidant capable of oxidizing manganate ions to permanganate ions during the treatment process resulting in excellent stability and increased operating life for the bath. The formation of undesirable lower oxidation state manganese products such as manganese dioxide is lowered and, additionally, the activity of the solution is also unexpectedly enhanced and shorter treatment times are generally required than with prior art compositions.
The demands of industry are always increasing however, and there is still a need for improved alkaline permanganate etching solutions and methods for using the solutions to etch plastic substrates.